Fix the regex ingredient being incredibly borked. (#758)

It will now actually serialize and deserialize using the same key, and it will actually serialize the full pattern and flags, instead of just the pattern (which can't be understood by the method used to parse it)

forge/src/main/java/dev/latvian/mods/kubejs/platform/forge/ingredient/RegExIngredient.java

@@ -16,6 +16,7 @@ public class RegExIngredient extends KubeJSIngredient {
 	public final Pattern pattern;
 
 	public RegExIngredient(Pattern pattern) {
+		if (pattern == null) throw new IllegalArgumentException("Pattern for a RegExIngredient cannot be null! Check your pattern format");
 		this.pattern = pattern;
 	}
 
@@ -39,7 +40,7 @@ public boolean test(@Nullable ItemStack stack) {
 
 	@Override
 	public void toJson(JsonObject json) {
-		json.addProperty("regex", pattern.toString());
+		json.addProperty("pattern", UtilsJS.toRegexString(pattern));
 	}
 
 	@Override